1. Field of the Invention
This invention relates to leach solutions for leaching manganese oxide containing materials to recover metal values contained therein.
2. Description of the Prior Art
By using a geochemical prospecting process, it is possible to infer the presence, location and magnitude of ore bodies below the surface. This process involves detecting surface areas containing excess localized anomalous concentrations of ore metals in comparison with their normal concentrations in the region. Such excess may result from diffusion or capillary transport of metal ions upward through overburden, or from physical transport of ore minerals, or from other hydromorphic processes which transport ground water and resulting stream flows. A detailed description of such prospecting may be found in "Geochemistry in Mineral Exploration" (2nd Ed.) by Rose, Hawkes and Webb, Academic Press, New York, 1979.
In an illustrative hydromorphic process, manganese and iron ions, together with associated ore metal compounds, are transported by water from below ground to the surface, where they precipitate as oxides. The fresh, initially largely amorphous, oxides deposit as coatings upon available solid bodies, ranging from clay particles to rocks and boulders. These oxides in turn scavenge heavy metal ions and metal compounds dissolved in the water. Depending upon the deposit conditions, metals such as silver (Ag), cobalt (Co) and copper (Cu) will associate predominantly with manganese oxides. Lead (Pb) will associate with iron oxide, while zinc (Zn) and nickel (Ni) will associate with either. Several forms of manganese dioxide occur in geological materials. Most of the manganese dioxide (MnO.sub.2) in typical soils or sediments is present in crystalline phases. Amorphous and semi-amorphous (partially crystalline) manganese dioxide usually are a portion of the MnO.sub.2 in surficial geological materials. The amorphous form of this compound is the most reactive form, and it is a very effective trap for many trace elements due to its complex surface and larger surface area.
Because the concentration of ore metals is higher in the oxide coatings than in the underlying solid bodies, it is advantageous to strip these coatings off the substrate before testing them for their metal content. This step enhances the contrast between the composition of anomalous samples and that of the normal background in the region, and permits a more sensitive and extensive delineation of the anomalous area. It is further advantageous to be able to strip separately the manganese oxide and iron oxide metal-containing materials.
For this purpose, numerous selective leach solutions have been developed which, when applied separately or in carefully-ordered sequences in well-known partial dissolution techniques, can strip off one or more of the separate components of the coating and resolve the sample into useful fractions. Most processes dissolve all of the manganese oxide. See, T. T. Chao, "Use of Partial Dissolution Techniques in Geochemical Exploration," Journal of Geochemical Exploration, Vol. 20 (1984) pp. 101-135, Elsevier Science Publishers, Amsterdam, and the bibliography attached thereto. Among these known leach solutions are hydroxylamine hydrochloride, oxalic acid and ascorbic acid. Hydroxylamine hydrochloride contains chloride ions, which can produce serious analytical interferences, and is not a viable leaching agent when seeking many low-level trace-element signatures. Ascorbic acid and oxalic acid leaches, on the other hand, are not selective for certain oxides.
Oxalic acid leaching of rock, soil, and stream sediment samples as an anomaly-accentuation technique is described by H. V. Alminas and E. M. Mosier in U.S. Geological Survey Open File Report 76-275, 25 pp, 1976. A rapid partial leach and organic separation for the sensitive determination of Ag, Bi, Cd, Cu, Mo, Pb, Sb, and Zn in surface geologic materials by flame atomic absorption is described by J. G. Viets, J. R. Clark and W. L. Campbell in Journal of Geochemical Exploration, Vol. 20, p. 355-366, 1984.
While concentrated hydrogen peroxide has been widely used as an oxidant in selective leaching processes, particularly for the destruction of organic portions of the sample, hydrogen peroxide can also function as a reducing agent for several metallic oxides. In an aqueous solution, it will react with manganese dioxide, consuming hydrogen ions, resulting in the manganese being reduced to the divalent state, which is soluble, thus: EQU MnO.sub.2(s) +H.sub.2 O.sub.2 +2H.sup.+ .fwdarw.Mn.sup.2+ +O.sub.2(aq) +2H.sub.2 O
In the process, all trace elements trapped in the manganese dioxide are released. See, for example, U.S. Pat. No. 4,872,909, issued Oct. 10, 1989 to J. P. Allen, et al. for "Process for Acid Leaching of Manganese Oxide Ores Aided by Hydrogen Peroxide." It is known, however, that concentrated H.sub.2 O.sub.2 will only slowly dissolve some crystalline forms of manganese oxide.
Dilute hydrogen peroxide is a poor oxidizer of metallic gold and sulfide minerals. It is known, however, that H.sub.2 O.sub.2 will act in combination with other reagents to aid in the leaching of these substances. The presence of H.sub.2 O.sub.2 raises the fugacity of oxygen in a solution and helps increase the efficiency of dissolution of gold by cyanide. Hydrogen peroxide will oxidize halide ions, such as chloride, bromide, and iodide, to chlorine, bromine, and iodine. Aqueous solutions of these halogen elements can effectively oxidize and dissolve precious metals such as gold and many sulfide minerals.
It is well known in the art of beekeeping, that natural raw honey contains a very low concentration of the heat-labile enzyme, glucose oxidase, which, acting upon the dextrose in the honey, has the interesting property of maintaining a very low, 33 parts per million, concentration of hydrogen peroxide in the honey. This peroxide, among other factors, prevents growth of pathogenic and degradative microorganisms in the honey. In brief the reaction is: ##STR1##